1. Field of the Invention
This invention relates to an image forming apparatus based on the electrophotographic process applicable to printers, facsimiles and copying machines. More particularly, it relates to the image forming apparatus having an exposure means inside of a photoreceptor formed in a drum or endless belt, whereby the photoreceptor is developed as soon as the exposure with the exposure means.
2. Description of the Prior Art
There has been known an electrophotographic apparatus based on an electrophotographic process or the Carlson process to form image of which photoreceptor drum is disposed around a peripheral thereof with various process means for exposure, development, transfer, and cleaning or removing residual toner particles, erasing charge, and electrification.
Because the various process means are independently disposed around the peripheral of the photoreceptor drum, and because a high potential is required for electrification and biasing, the constitution of the apparatus has been rendered sophisticated and large in size.
To dissolve the issues, a prior technique has provided an image forming apparatus disclosed in Japanese Laid Open Provisional Application No. 58-153957 with a technique called as a rear side exposure system hereinafter. The apparatus is formed with a photoreceptor drum comprising transparent support member, laminating a transparent electroconductive layer and a photoconductive layer thereon. The apparatus also is disposed with an exposure means in the photoreceptor drum of which exposure means generates a light beam corresponding to image information. In the apparatus, the output light beams with the exposure means are focused to expose on the photoconductive layer through a convergence lens. Then soon after the exposure or simultaneously thereto, a latent image is developed into a toner image on the photoreceptor drum which is opposed with the toner support member. Finally, the toner image is transferred on recording paper with a transfer means of transfer rollers, or the like.
In the apparatus of this kind unlike to the traditional Carlson system electrophotographic apparatus of which exposure means is disposed outside of the photoreceptor drum, it is hard to adopt an exposure means which dissipates the light beams with polygon mirror or the like, because the exposure means is disposed inside of the limited space of photoreceptor drum. To dissolve the issue, another prior technique is disclosed in Japanese Laid Open Provisional Application No. 63-142383, in which an apparatus is equipped with an exposure means disposed with a plurality of LED elements in an array along the drum axially thereto, wherein the LED elements are controlled selectively to light corresponding to image information. Another prior technique is also disclosed in Japanese Laid Open Provisional Application No. 62-280772, in which an apparatus is equipped with a liquid crystal shutter disposed between a light source and a convergence lens, wherein the exposure image is formed with the liquid crystal shutter which is controlled to open and close. Still another prior technique is disclosed in Japanese Laid Open Provisional Application No. 62-280773, in which an exposure means is formed by EL head laid with electroluminescence elements in an array.
Several issues, however, are involved in the liquid crystal head, which is restricted within a narrow temperature range to work, which requires an additional light source, and which is limited within a slow processing speed, because of a slow response speed thereof, and because of a small contrast between dark and light tone thereof.
The electroluminescence elements also reveal an issue of which luminescent intensity is weaker than that of LED elements, or the like. Unlike the Carlson system, as described earlier, which exposes the light beam directly on the photoconductive layer, the rear side exposure system has to expose the beam through the transparent support member and the transparent conductive layer. In the event of the weaker luminescent intensity, the less photoactivated charge becomes the photoconductive layer, because the beam has to penetrate the barriers of transparent support member and transparent photoconductive layer. Thus the weak luminescent intensity of the elements becomes a fatal issue to form an intensified image.
The LED head, therefore, is advantageously preferred currently to form the intensified image with a moderate image processing speed.
With the LED head, further, during the process for forming a latent image corresponding to image information illuminating output light beams on the photoreceptor drum, it is possible to strengthen the intensity of the light beam with the enhanced image intensity and sharpness keeping the appropriate image processing speed, if the LED head is applied a large drive electric current. Thus, the LED head gives another advantage.
Care must be taken to adopt the LED head, for example, of which basic constitution is formed as to array n pieces of 64 bits LED tips in a line. It is necessary that 40 pieces of LED tips to print on recording paper having an A4 size width with a pixel density 300 dots/inch (dpi), or approximately 12 dots/mm. It has, then, to control a large current for forming image, if the all set of 64.times.40 pieces of LED elements are attempted to light on simultaneously for the line. The large current which requires a big power unit renders the constitution of apparatus large in size.
It also renders the joule heat increased to light the large number of LED elements simultaneously for the line pixels. The generation of heat results in the wave length and light intensity of the LED elements fluctuated, of which characteristics strongly depend on the temperature.
As described earlier, the LED head is expected to be disposed in the almost enclosed small space of photoreceptor drum. The temperature in the photoreceptor drum is easily raised, if the exposure process is done in such a manner that the heat generating unit is inserted in the small space of drum. The raised temperature results in varying the dark resistivity and the electron velocity in the photoconductive layer, which exerts an undesirable influence on the image quality. If the attempt is further carried to enclose the space for preventing dust from invasion in the drum at the both ends thereof, the rise of temperature is more intensified to stress mischievously the issue.
Further, it is required for more lead wires corresponding to the large number of LED elements to light the large number of LED elements simultaneously for the full pixel line. The large number of wirings requires a larger space for the LED head in the drum, of which space increases the sectional area of the LED head resulting in abandoning the attempt for a drum in a smaller size. More practically, the issues have restricted the size of photoreceptor drum not to be less than 50 mm diameter.
To dissolve the issues, instead of the LED head driven statically for lightening the LED elements simultaneously for the full pixel line, the inventor has developed an LED head driven dynamically for exposing subsequently the full pixel line in block by block of which pixel line is divided into blocks in a tip unit or an appropriate number n of elements. Each of units is driven one after the other in a time sharing manner.
It is sure that a technique for dynamic drive LED head adopted for the Carlson system is disclosed as in Japanese Laid Open Provisional Application No. 60-34877 and so forth. No prior technique, however, is disclosed, nor information is available for the dynamic drive LED head adopted for the rear side exposure system as in the present invention.
The reason for the no prior technique may be lies in how to form the intensified and sharpened image.
Because, as described earlier, the dynamic drive LED head exposes light beam subsequently the blocks in the time sharing manner within a cycle time for the pixel line, the exposure time for each of blocks must be reduced comparing with that of the static drive LED head. In the apparatus, further, charge photoactivated in the photoconductive layer is rendered small, because the light beam is exposed at the rear side of the photoreceptor through the transparent support member and the transparent electroconductive layer. Thus, it has been impossible to form the image with higher intensity and sharpness.
In the traditional apparatus, therefore, it seems to refrain from adopting the dynamic drive LED head for the rear side exposure system, because no exposure light intensity enough to form the clear image has been available.